R1:70fd96037bfd

Consolidate user editing code

Summary:
- We currently have some bugs in account creation due to nontransactional user/email editing.
- We save $user, then try to save $email. This may fail for various reasons, commonly because the email isn't unique.
- This leaves us with a $user with no email.
- Also, logging of edits is somewhat inconsistent across various edit mechanisms.
- Move all editing to a `PhabricatorUserEditor` class.
- Handle some broken-data cases more gracefully.

Test Plan…